Abstract: This paper discusses fundamental assumptionand simplification of the thin-layer element. The thin-layer element constitutive model can be obtainedfrom the constitutive model of transversely isotropic materials.The constitutive relation of thin-layer elements in crak can be obtainedby using the distributed crack model. The fundamental assumption of thin-layer elements can assurecalculation accuracy and simplify calculation to a certain extent.